Sorghum is a genus of about 20 species of grasses native to tropical and subtropical regions of Eastern Africa, with one species native to Mexico. Sorghum is cultivated in Southern Europe, Central and North America and Southern Asia. Sorghum is also known as Durra, Egyptian Millet, Feterita, Guinea Corn, Jowar, Juwar, Kaffircorn, Milo and Shallu. Sorghum is used for food, fodder and the production of alcoholic beverages. It is an important food crop in Africa, Central America and South Asia, especially for subsistence farmers. It is used to make such foods as couscous, sorghum flour, porridge and molasses. The leading producer of sorghum is the United States where it is primarily used as a maize substitute for livestock feed because the nutritional content of sorghum and maize is similar. Sorghum is usually used as a lower cost substitute for maize in livestock rations. Sorghum is also used to make ethanol and other industrial products.
Sorghum is in the same family as maize and has a similar growth habit, but with more tillers and a more extensively branched root system. Sorghum is more drought-resistant and heat-tolerant than maize. It requires an average temperature of at least 25° C. to produce maximum yields. Sorghum's ability to thrive with less water than maize may be due to its ability to hold water in its foliage better than maize. Sorghum has a waxy coating on its leaves and stems which helps to keep water in the plant even in intense heat. Wild species of sorghum tend to grow to a height of 1.5 to 2 meters, however in order to improve harvestability, dwarfing genes have been selected in cultivated varieties and hybrids such that most cultivated varieties and hybrids grow to between 60 and 120 cm tall. It is commonly accepted that there are four dwarfing genes in sorghum. 
Hybrid production in sorghum is accomplished by crossing a female line (cytoplasmic male sterile line derived from non-restorer germplasm) with a male line containing a restorer gene. Several sorghum restorer genes have been identified through mapping. Klein, et al., (2001) Theor. Appl. Genet. 102:1206-1212 have mapped Rf1 gene on LG-H (LG-08) for A1 type cytoplasm. Wen, et al., (2002) Theor. Appl. Genet. 104:577-585 have mapped Rf4 gene in A3 type cytoplasm. Tang, et al., (1996) Plant J. 10:123-133 and Tang, et al., (1998) Genetics 150:383-391 have mapped the Rf3 gene in A3 type cytoplasm.
Germplasm carrying a restorer gene is numerous and diverse. Developing males (restorers) takes relatively less effort than developing females. As a result, both private and public breeding programs have focused on development of male lines that carry a restorer gene. The pool of available non-restorer (female) germplasm is less diverse and receives less attention in the public sectors. Within private industry, considerable resources are devoted to developing non-restorer germplasm but this activity is limited by both the pool of available non-restorer germplasm and the need for confirming non-restorers by test-crossing with restorer lines and evaluating subsequent hybrids. Currently, breeders confine themselves to making largely restorer-by-restorer or non-restorer by non-restorer crosses and rarely make non-restorer by restorer crosses because of the tedious procedure of separating restorers and non-restorers in subsequent generations as well as the unpredictability of the results. Facilitating such crosses using a marker associated with a restorer gene would enhance the breeders' ability to diversify the germplasm base of the non-restorer population leading to enhanced genetic progress and improved inbreds and hybrids. A marker for a restorer gene would also allow breeders to use marker-assisted selection and to more rapidly phenotype germplasm with unknown restoration reaction allowing new germplasm to efficiently flow into the restorer and non-restorer germplasm pools.